As oil reserves become ever more scarce and difficult to find, improved information on the downhole environment is becoming increasingly crucial to successful exploration and production. Knowledge of downhole temperature fluctuations helps to identify materials and flow rates downhole, and can thus be a particularly promising data set. Under certain temperature and/or pressure conditions, gas hydrates are known to dissociate, form, or be otherwise affected. Better understanding of the parameters of such behavior in the downhole environment will be an important advance toward more efficient exploration and development of underground resources. Specifically, such information would prove very helpful in developing gas hydrates and heavy oils as energy resources. Therefore, collecting and analyzing thermal measurements is important in both static and dynamic characterization of subsurface structures.
The stability of formation fluids, including hydrocarbons such as gas hydrates and heavy oils, is sensitive to variations in pressure and temperature. In this regard, gas hydrates dissociate or form when pressure and/or temperature conditions cross equilibrium borders. Conventional methods to model the thermal properties of subsurface formations (e.g., thermal conductivity, diffusivity and capacity measurements) require taking passive temperature measurements at several underground locations, and then performing core analysis in a laboratory (attempting to duplicate the downhole conditions in the laboratory) and comparing the results. However, laboratory core analysis of hydrate bearing zones is often difficult as the downhole conditions are not easily replicated. Further, the accuracy of the laboratory model is dependent upon factors such as coring conditions due to the dynamic dissociation/formation process of hydrates downhole, compaction factor differences under different pressures, and sometimes upon extrapolating data from missing core samples at certain depth intervals. In addition, when using passive temperature measurements taken at several underground locations downhole, the acquired data are interpreted under certain assumptions that increase the uncertainties (e.g., that heat flow is steady, that thermal disturbances from drilling or mud circulation are negligible).
Several commercial distributed-temperature-sensor (DTS) applications are available that give temperature measurements along the entire length of a cable for fire detection and temperature control in warehouses, and include high-temperature-resistant DTS cables. Such DTS cables consist of fiber-optic loops and use a variety of scattering phenomenon to sense the temperature along the length of the fiber-optic loops. The scattering phenomenon in optical fibers arise because temperature, pressure and tensile forces cause changes to the light frequency scattered by the optical fiber. Such scattering effects include Brillouin scattering, which results from temperature variations and strain within the optical fiber, and Raman scattering, which results only from temperature variations in the optical fiber.
Commercially available DTS equipment is useful in laboratory core analysis, however, applying laboratory-based active temperature measurement methods to in-situ subsurface formation measurement leads to many technical and logistical difficulties. In one example, a DTS cable is lowered into the flow of the completed well and continuous measurements are taken. However, this method reduces flow from the well because the DTS cable is obstructing a portion of the wellbore. Also, temperature measurements are highly influenced by the temperature of the material flowing by, and have less correlation to the property of the formation, itself.
In addition to the desire to accurately measure the temperature along the entire wellbore for better modeling of the subsurface formations, it is sometimes desirable to manipulate the formation fluids, by introducing heat into the wellbore to effect the dissociation or formation of hydrates. This is typically done by lowering a heating element into the wellbore and heating the formation adjacent to the heating element to induce the desired hydrate formation or dissociation. However, this introduces additional obstructions into the wellbore.
From the foregoing it will be apparent that there is a need for apparatus for introducing heat into subsurface formations, for example, for the use with distributed-temperature-sensor cable for subsurface modeling. An apparatus that combines these features would provide additional advantages.